This invention concerns a process for regenerating certain palladium catalysts used in the carbonylation of olefins. Carbonylation refers here to the reaction of .alpha.-olefins with carbon monoxide and active-hydrogen-containing compounds selected from the group consisting of alkanols or water. The major products of carbonylation are fatty (carboxylic) acids and their esters.
The preparation of the fatty acids or fatty acid esters using metal carbonyls or carbonyl precursors to catalyze the carbonylation of olefins is old in the literature, originally involving Reppe and his coworkers and contemporaries. Reviews of C. W. Bird [Chem. Rev. 62, (1962)] document this work. Unfortunately, many of these carbonyl or carbonyl-type catalysts have the disadvantages of inherent toxicity, they require stringent reaction conditions which in turn lead to competing side reactions such as olefin isomerization, polymerization and reduction, and they exhibit poor selectivity to the desired linear acid ester.
Recently, more acceptable homogeneous catalyst systems have been developed which offer substantially improved selectivity in converting olefins to primarily linear fatty acids or linear fatty esters, in good yield, under moderate reaction conditions of temperature and pressure.
As is usually the case, after much more extensive usage, certain drawbacks in the catalysts have become more evident. These include difficulty in maintaining high conversions, high selectivities and high yields after recycling the catalyst several times. These problems are due to catalyst degradation as well as catalyst decomposition, mechanical losses and further catalyst decomposition during the separation of the products from the homogeneous catalysts and the inert solvents of the reaction mixture. Thermal instability of the catalyst is particularly troublesome in the recovery and working-up of certain ligand-stabilized homogeneous palladium catalyst reaction mixtures.
In order to avoid or minimize these problems, the use of molten quaternary ammonium, phosphonium and arsonium salts of trihalostannate(II) and trihalogermanate(II) as both solvent and part of the catalytic entity has been disclosed, particularly in the two United States patents of G. W. Parshall, U.S. Pat. Nos. 3,657,368 and 3,565,823, which are known art as well as in applicant's Serial No. 526,867 filed 11/25/74 in the United States Patent Office. More recently, two procedures involving distillation and solvent extraction as a means of isolating fatty aced/ester products from palladium carbonylation catalysts consisting of dispersions of ligand-stabilized palladium(II) halides in quaternary ammonium, phosphonium and arsonium salts of trihalostannate(II) and trihalogermanate(II) have been set forth in Ser. No. 581,320 and Ser. No. 581,395 both filed May 27, 1975. Following the separation of catalyst and fatty acid ester product by these methods the palladium catalyst is suitable for recycle with fresh olefin/alcohol feed. Regeneration of the same palladium catalysts after multiple cycling by chlorination or mineral acid treatment is also set forth in applications Ser. No. 581,320 and 581,395. Each of these process improvements allows the development of an efficient carbonylation process having the advantages of:
1. a simple and efficient means of separating catalyst from the products,
2. the ability to recycle the catalyst without its substantial deactivation. This is particularly important since the palladium catalysts are thermally sensitive, and
3. a capability for operating at high catalyst concentrations in the feed stream, thereby minimizing capital costs.